A typical conventional method for producing an air-free package uses either an end-seal or side-seal pre-formed bag made of a thermoplastic material.
In both cases, a product to be packaged is loaded into the pre-formed bag, the bag is evacuated in a vacuum chamber, and the open mouth of the vacuumized bag is then sealed or clipped.
Very often this method is used in conjunction with a pre-formed bag made of a heat-shrinkable thermoplastic material. After sealing or clipping of the vacuumized bag, the bag is heated while traveling through a shrink tunnel. The bag thus shrinks around the packaged product.
In the case of a food product, the thus obtained air-free package increases the shelf-life of the packaged product.
This method, however, lacks flexibility.
A first reason is that only some standard sizes of bags are available that do not fit the size of every product to be packaged.
Secondly, the packaging machine has to be set on the size of the bag which is being used. When a change in size is required, the packaging machine has to be shut-down, reset, and restarted.
Thirdly, high packaging speed cannot be achieved even if the final steps of the packaging process (i.e. vacuumizing, sealing, and shrinking) are performed in a revolving machine equipped with a plurality of vacuum and sealing chambers. The earlier steps (i.e., opening of the bag mouth and loading of the product) cannot be sped up without risking an unacceptable increase in rejects.
Finally, the storage of pre-formed bags having many different sizes is costly.
In order to overcome these drawbacks it has been proposed, in U.S. Pat. Nos. 3,237,371, 4,141,196, and 4,537,016, to form a bag in line from film webs which are continuously shaped into a tubular form. However, this also lacks flexibility. Although the width of the package can be adjusted depending on the width of the product to be packaged by means of suitable sensors that provide a proper input to a microprocessor, the diameter of the tube (i.e., the length of the package) cannot be changed in accordance with the length of the product itself.
It is therefore desirable to provide a method and apparatus capable of automatically forming packages, including air-free packages, from a continuous thermoplastic film wherein any single resulting package, including a tight-skinned package, can have a different length and/or width without requiring any resetting of the apparatus.